Microporous polyolefin membranes are used in various applications such as battery separators, electrolytic capacitor separators, various filters, steam-permeable, water-proof clothing, reverse osmosis filtration membranes, ultrafiltration membranes, microfiltration membranes, etc. When the microporous polyolefin membranes are used as battery separators, particularly lithium ion battery separators, their performance largely affects the characteristics, productivity and safety of batteries. Accordingly, the microporous polyolefin membranes are required to have excellent permeability, mechanical properties, heat shrinkage resistance, shutdown properties, meltdown properties, etc. For instance, when microporous polyolefin membranes having low mechanical strength are used as battery separators, battery voltage soon becomes low.
To improve the properties of microporous polyolefin membranes, the optimization of starting material compositions, stretching conditions, heat treatment conditions, etc. have been proposed. For instance, JP 2-94356 A proposes a microporous polyethylene membrane for lithium battery separators having good assemblability and low electric resistance, which is produced by melt-blending a high-density polyethylene resin having a mass-average molecular weight (Mw) of 400,000 to 2,000,000 and a molecular weight distribution [mass-average molecular weight/number-average molecular weight (Mw/Mn)] of 25 or less with fine inorganic powder and an organic liquid, extruding the resultant melt blend through a die, cooling the resultant extrudate to provide a gel-like sheet, removing the fine inorganic powder and the organic liquid, and stretching the resultant membrane to 1.5-fold. However, this microporous polyethylene membrane has insufficient strength because of too large surface pore size.
JP 5-9332 A proposes a microporous membrane having high strength and a proper pore size, which is produced by melt-blending ultra-high-molecular-weight polyethylene having a viscosity-average molecular weight of 2,000,000 or more with fine inorganic powder and a plasticizer, extruding the resultant melt blend through a die, cooling the resultant extrudate to provide a gel-like sheet, removing the fine inorganic powder and the plasticizer, drying the resultant membrane, and stretching the membrane only in one direction. However, this microporous membrane also has insufficient strength because of too large surface pore size.
In such circumstances, the applicant proposed a microporous polyolefin membrane made of a polyolefin composition comprising 1% or more by mass of a component having Mw of 7×105 or more, and having Mw/Mn of 10 to 300, whose degree of orientation changes in a thickness direction (Japanese Patent 3347854). This microporous polyolefin membrane having excellent mechanical strength is produced by melt-blending the above polyolefin composition and a membrane-forming solvent, extruding the resultant melt blend through a die, cooling the resultant extrudate to provide a gel-like sheet, stretching the gel-like sheet while heating to provide a temperature distribution in a thickness direction, and removing the membrane-forming solvent.
The applicant also proposed a microporous polyolefin membrane constituted by fine fibrils made of a polyolefin having Mw of 5×105 or more or a polyolefin composition containing such polyolefin, which has an average pore size of 0.05 to 5 μm, the percentage of crystal lamellas having angles θ of 80 to 100° relative to a membrane surface being 40% or more in each longitudinal or transverse cross section (WO 2000/20492). This microporous membrane having excellent permeability is produced by melt-blending 10 to 50% by mass of the above polyolefin or polyolefin composition with 50 to 90% by mass of a membrane-forming solvent, extruding the resultant solution through a die, cooling the resultant extrudate to provide a gel-like sheet, stretching the gel-like molding, if necessary, heat-setting the resultant membrane at a temperature in a range from the crystal dispersion temperature of the polyolefin or polyolefin composition to its melting point +30° C., and removing the membrane-forming solvent.
The applicant also proposed a microporous polyolefin membrane made of a polyolefin having Mw of 5×105 or more or a polyolefin composition containing such polyolefin, in which an average pore size gradually decreases from at least one surface to a center in a thickness direction (WO 2000/20493). This microporous membrane having excellent permeability is produced by melt-blending 10 to 50% by mass of the above polyolefin or polyolefin composition with 50 to 90% by mass of a membrane-forming solvent, extruding the resultant solution through a die, cooling the resultant extrudate to provide a gel-like sheet, and bringing the gel-like sheet into contact with a hot solvent and then removing the membrane-forming solvent, or removing the membrane-forming solvent from the gel-like sheet and then bringing the resultant membrane into contact with a hot solvent.
However, recently gaining importance as separator characteristics are not only permeability and mechanical strength, but also battery life properties such as cycle properties and battery productivity properties such as electrolytic solution absorbability. Particularly a lithium ion battery electrode expands by the intrusion of lithium when charged, and shrinks by the departure of lithium when discharged, an expansion ratio when charged tending to become larger as recent increase in the capacity of batteries. Because a separator is compressed when the electrode expands, the separator is required to be deformable to absorb the expansion of an electrode while suffering only small variation of permeability by compression. However, any microporous membrane described in the above references does not have sufficient compression resistance. A microporous membrane with poor compression resistance is highly likely to provide batteries with insufficient capacity (poor cycle properties) when used as a separator.
Poor electrolytic solution absorbability leads to the poor productivity of batteries. To improve the electrolytic solution absorbability, it is effective to provide a separator with a large pore size on the surface. Also, to prevent the clogging of a separator with by-products formed by the repetition of charge/discharge cycles, the separator is required to have large pore size on the surface. To ensure enough mechanical strength, however, a layer having a dense structure is needed. Thus, the separator is desired to comprise a coarse-structure layer having a relatively large average pore diameter on at least one surface, and a layer having a dense structure.